1. Field of the Invention
The present invention relates to an augmentation-index determining apparatus which determines an augmentation index of a living subject, and an arteriosclerosis inspecting apparatus which inspects arteriosclerosis of a living subject based on an augmentation index of the subject.
2. Related Art Statement
Augmentation index, generally known as AI, indicates a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, and is used to evaluate compliance of aorta. The greater the compliance is, the smaller the reflected-wave component is, and the smaller the compliance is, the greater the reflected-wave component is. That is, the harder the aorta is, the greater the reflected-wave component of the aortic pulse wave is. Thus, the augmentation index indicates arteriosclerosis and accordingly is used as an index to examine or inspect it.
As described above, augmentation index represents a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same. However, since it is difficult to completely separate a detected pulse wave into an incident-wave component and a reflected-wave component, an augmentation index is determined by first identifying respective times of occurrence of respective peak points of the incident-wave and reflected-wave components of the detected pulse wave, and then dividing a difference between respective magnitudes of the detected pulse wave at the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components, by a pulse pressure of the detected pulse wave. A time of occurrence of an inflection point or a maximal point of the detected pulse wave between a rising point and a peak point of a heartbeat-synchronous pulse of the detected pulse wave is identified as the time of occurrence of the peak point of the incident-wave component; a time of occurrence of the first or earliest maximal point of the heartbeat-synchronous pulse of the detected pulse wave, subsequent to the peak point of the incident-wave component, is identified as the time of occurrence of the peak point of the reflected-wave component; and a difference between the greatest and smallest magnitudes of the heartbeat-synchronous pulse of the detected pulse wave is determined as the pulse pressure of the detected pulse wave.
As described above, augmentation index is used to evaluate compliance of aorta. To this end, clinically, augmentation index is determined based on a carotid pulse wave that is detected from carotid artery that is the nearest to the aorta of all the arteries from which pulse waves can be non-invasively detected.
However, skill is needed to wear a carotid-pulse-wave sensor at an appropriate position to detect a carotid pulse wave, and a considerably long time is needed to detect it. Thus, there has been a demand to determine augmentation index based on a more easily detectable pulse wave, such as a brachial artery.
However, it has been elucidated that regarding the other sorts of pulse waves than the carotid pulse wave, there are some cases where a time of occurrence of a peak point of a reflected-wave component does not correspond to the earliest maximal point subsequent to a peak point of an incident-wave component.
Each of FIGS. 1A and 1B shows, regarding a corresponding one of two patients, a carotid pulse wave and a brachial pulse wave that are simultaneously detected from the one patient and respective rising points of which are aligned with each other. The brachial pulse wave is extracted from pressure oscillation that is transmitted to a cuff in a state in which a pressure of the cuff is lower than a diastolic blood pressure of the patient (in this specification, this pulse wave will be referred to as the xe2x80x9clow-cuff-pressure pulse wavexe2x80x9d). This is a common method to detect an accurate pulse wave.
Regarding the case shown in FIG. 1A, a time, t1, of occurrence of a peak point of an incident-wave component and a time, t2, of occurrence of a peak point of a reflected-wave component, each determined on a brachial pulse wave, coincide with those determined on a carotid pulse wave. In many cases, this is true.
On the other hand, regarding the case shown in FIG. 1B, a time t1 of occurrence of a peak point of an incident-wave component determined on a brachial pulse wave coincides with that determined on a carotid pulse wave. However, a time t3 of occurrence of a peak point of a reflected-wave component of the brachial pulse wave, i.e., a time of occurrence of the earliest maximal point subsequent to the peak point of the incident-wave component, does not coincide with a time t2 of occurrence of a peak point of a reflected-wave component of the carotid pulse wave. Therefore, if an augmentation index is determined based on the brachial pulse wave shown in FIG. 1B in the conventional method, then the index is not accurate.
It is therefore an object of the present invention to provide an augmentation-index determining apparatus which can determine an augmentation index with high accuracy.
The Inventors have found that pressure oscillation occurring to a cuff in a state in which a pressure of the cuff is higher than a systolic blood pressure of a living subject has two maximal points, as will be described later in connection with FIGS. 4A and 4B, and that the first maximal point corresponds to a peak point of an incident-wave component and the second maximal point corresponds to a peak point of a reflected-wave component. If respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components are used to determine respective times of occurrence of respective peak points of incident-wave and reflected-wave components of a pulse wave detected to determine an augmentation index, then the index can enjoy high accuracy. The present invention has been developed based on this concept.
The above object has been achieved by the present invention. According to the present invention, there is provided an apparatus for determining, based on a pulse wave detected from a living subject, an augmentation index indicating a proportion of a reflected-wave component of the pulse wave to an incident-wave component thereof, the apparatus comprising an inflatable cuff which is adapted to be worn on a portion of the subject and inflated to apply a pressing pressure to the portion; a cuff-pressure changing device which changes the pressing pressure of the cuff; a pulse-wave extracting device which extracts a pulse wave from a pressure oscillation which is transmitted from the portion of the subject to the cuff; a peak-occurrence-time determining means for determining, based on a high-cuff-pressure pulse wave which is extracted by the pulse-wave extracting device when the cuff-pressure changing device makes the pressing pressure of the cuff higher than a systolic blood pressure of the portion of the subject, a time of occurrence of a peak point of an incident-wave component of the high-cuff-pressure pulse and a time of occurrence of a peak point of a reflected-wave component of the high-cuff-pressure pulse wave; and an augmentation-index determining means for determining, based on the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components of the high-cuff-pressure pulse, determined by the peak-occurrence-time determining means, respective times of occurrence of respective peak points of incident-wave and reflected-wave components of a low-cuff-pressure pulse which is extracted by the pulse-wave extracting device when the cuff-pressure changing device makes the pressing pressure of the cuff lower than a mean blood pressure of the portion of the subject, and determining an augmentation index based on respective magnitudes of the low-cuff-pressure pulse wave at the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components of the low-cuff-pressure pulse.
According to this invention, the peak-occurrence-time determining means determines, based on the pulse wave detected when the pressing pressure of the cuff is higher than the systolic blood pressure of the subject, the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components of the pulse wave, and the augmentation-index determining means determines, based on the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components, determined by the peak-occurrence-time determining means, the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components of the low-cuff-pressure pulse, and determines the augmentation index based on the respective magnitudes of the low-cuff-pressure pulse wave at the respective times of occurrence of the respective peak points of the incident-wave and reflected-wave components of the low-cuff-pressure pulse. The thus determined augmentation index enjoys a high accuracy.
According to a preferred feature of the present invention, the cuff is adapted to be worn on an upper arm of the subject, the augmentation-index determining means determines the augmentation index based the high-cuff-pressure pulse wave and the low-cuff-pressure pulse wave each of which is extracted by the pulse-wave extracting device from the pressure oscillation transmitted from an artery of the upper arm to the cuff worn on the upper arm.
According to another preferred feature of the present invention, the cuff-pressure changing device decreases, in a blood-pressure measuring operation, the pressing pressure of the cuff from a pressure higher than the systolic blood pressure of the portion of the subject, to a pressure lower than a diastolic blood pressure of the portion of the subject, the pulse-wave extracting device extracts the high-cuff-pressure pulse wave when the cuff-pressure changing device makes, in the blood-pressure measuring operation, the pressing pressure of the cuff higher than the systolic blood pressure of the portion of the subject, and the pulse-wave extracting device extracts the low-cuff-pressure pulse wave when the cuff-pressure changing device makes, in the blood-pressure measuring operation, the pressing pressure of the cuff lower than the diastolic systolic blood pressure of the portion of the subject.
According to this feature, when the pressing pressure of the cuff is changed in the blood-pressure measuring operation, the high-cuff-pressure pulse wave and the low-cuff-pressure pulse wave are obtained, and the augmentation index is determined based on the high-cuff-pressure and low-cuff-pressure pulse waves. Thus, the augmentation index can be obtained concurrently with the blood-pressure measuring operation.
The above-indicated augmentation-index determining apparatus may be used as an arteriosclerosis inspecting apparatus. That is, according to another preferred feature of the present invention, there is provided an apparatus for inspecting arteriosclerosis of the living subject, based on the augmentation index determined by the augmentation-index determining apparatus.